The present invention relates generally to power control systems and more particularly to power control systems and methods for use with wireless communications system.
Wireless communications are becoming more popular as technology develops. Many people are now communicating through cellular telephones, pagers, radio frequency devices and others. As this trend continues, there is an ongoing need to optimize the communication channel between the two remote communications units.
One issue that arises is the power level that a mobile unit should use to transmit to the base station. In general, the base station has a desired signal level or range of signal levels for incoming communications. This level is determined by, among other things, the output power used by the mobile unit. Therefore, the mobile unit must be able to transmit at a power level compatible with the base station. This level will vary as the mobile unit moves closer to and farther from the base station.
In a well known method, the mobile unit will transmit a first message to the base station at a very low signal level. It will then keep raising the level until a response is received from the base station. The reason the mobile unit generally starts at a low level is because if the power level gets too high, the system will lose capacity. After the terminal begins communicating with the base station, the base station may tell the terminal to increase or decrease its power output. With CDMA (code division multiple access), for example, a continuous update signal is provided every millisecond so that the terminal adjusts its output power once a millisecond.
The preferred embodiment of the present invention discloses a power control method that utilizes both open loop control and closed loop control. An advantageous feature of the preferred embodiment is the ability to automatically control the output gain of the terminal units based upon the strength of the signal received at the base unit. This control can also, or alternatively, be based on the system gain of the base unit and/or a power level increment/decrement signal provided by the base unit.
In one aspect, the present invention describes a method of communicating from a terminal unit to a base unit. An incoming signal is sent from the base unit and received at the terminal unit. The terminal unit will measure the level of this incoming signal. The terminal unit may also extract digital information from the incoming signal. Based on either the measured signal level, the extracted information or both, the terminal will calculate an output power level to use in transmitting a signal back to the base unit. An outgoing signal can then be transmitted from the terminal unit using the output power level.
In another aspect, the present invention provides a method of communicating from a base unit to a terminal unit. In this method, which can be used in conjunction with the method described in the previous paragraph, the base unit transmits a system gain message to the terminal unit. This base unit system gain message includes information regarding the base power output level and/or the desired level of signal to be received from that terminal. At a later time, the base unit will receive a signal from the terminal unit and determine the signal level of that signal. Based on that signal level, the base unit will determined a power level increment factor and communicate that factor back to the terminal. The terminal can then use the power level increment factor to adjust its output power for future communications.
The power control techniques of the present invention are particularly useful in systems where the base unit is communicating with multiple terminal units over an FDM (frequency division multiplexed) spectrum, especially a system using sub-bands. In this application, it is important that out-of-band transmissions from one channel or subchannel do not interfere with the base units ability to receive transmissions from sources utilizing adjacent channels or subchannels. To accomplish this goal, it is useful if transmissions from each source are received at about the same signal level. The present invention provides a technique that helps to achieve this objective.
The output gain control functions also provide the advantage of increasing the battery life of the terminal units, as the base unit will continuously monitor the received signal and will instruct the terminal unit to only provide as much output gain as is necessary for signal clarity. In this way, the power consumed by the output circuitry of the terminal unit is minimized.